X-ray tube system with voltage and current control means



June 13, 1967 w. E. SPLAIN 3,325,645

x-RAY TUBE SYSTEM WITH VOLTAGE AND CURRENT CONTROL MEANS Filed Aug. 11, 1964 4 Sheets-Sheet 2 GRID I25 PM? l I I l I I I I l I l l I I l l I I I I I I I J WALTER E. SPLAIN FlG.-4 ,Lj-

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June 13, 1967 w. E. SPLAIN X-RAY TUBE SYSTEM WITH VOLTAGE AND CURRENT CONTROL MEANS Filed Aug. 11, 1964 I l +aov FIG 6 Is 10 Z t I25 I5l I55 \1 In If I 23s I 'M, so 43 22m :38 eas -9v 223 T0 MA STAB. I39

4 Sheets-Sheet 3 INVENTOR.

WALTER E. SPLAIN ATTORNEYS United States Patent 3,325,645 X-RAY TUBE SYSTEM WITH VOLTAGE AND CURRENT CONTROL MEANS Walter E. Splain, North Olmsted, Ohio, assignor t0 P cker X-Ray Corporation Waite Manufacturing Division,

Inc., Cleveland, Ohio, a corporation of Ohio Filed Aug. 11, 1964, Ser. No. 388,882 26 Claims. (Cl. 250-403) The present invention relates to X-ray systems and more particularly to a system for providing a substantially pure DC energizing voltage to an X-ray tube and for stabilizing X-ray tube voltage and tube current at preselected values.

To achieve maximum efiiciency and performance in X-ray tube systems, it is desirable that the high voltage supplied to the X-ray tube be as pure DC as possible. It is also desirable that the supplied voltage be stabilized and maintained at a preselected load independent of line and other changes in the system.

Prior attempts at voltage stabilization have attempted to stabilize primary voltage. Primary voltage stabilization has been accomplished in prior systems by a stabilizer and a stabilizer control. The stabilizer and stabilizer control are both located in the primary winding circuit of the high voltage transformer supplying voltage to the X-ray tube. This type of voltage stabilization is accomplished by sensing changes in the primary voltage and adjusting the primary voltage t-owmaintain it at a predetermined level. These prior systems inherently ignore changes in the actual energizing voltage supplied to the X-ray tube which are not accompanied by a corresponding change in the primary voltage. In addition, many of the prior systems are unduly complicated requiring a great number of components and frequent servicing.

In X-ray systems it is also desirable that all fluctuations or ripple be eliminated from the voltage supplied to. the X-ray tube. Prior attempts at eliminating the ripplein the rectified voltage supplied to the X-ray tube have generally incorporated some smoothing or filtering device. A capacitor is commonly used and is placed across the DC output terminals of the high voltage transformerrectifier circuit to smooth out the pulsating DC voltage by means of its filtering action. Although the capacitor does smooth out the pulsating DC voltage, substantial peak-to-peak ripple still remains in the energizing voltage supplied to the X-ray tube, when normal load, currents are drawn.

For example, in commercial full wave rectifier systems having a capacitive filter, the remaining ripple is approximately five kilovolts for a fifty kilovolt energizing voltage impressed across the X-ray tube. Where half-wave rectified voltage is supplied to the X-ray tube, the remaining ripple after the filtering action of the capacitor is often as much as eight kilovolts.

Another problem with prior systems is that in halfwave rectified voltage systems wherein two X-ray tubes are energized simultaneously from the secondary windings of a high voltage transformer, it has not been previously practicable to use the conventional capacitive filters to smooth out the pulsating half-wave rectified pulsating DC voltages. This is because the capacitive filters, when used, create large surge currents and unstable conditions within the X-ray system so that the half-wave rectified voltages supplied to X-ray tubes are not sufliciently controllable.

In the present X-ray system, the energizing voltage sup- X-ray tube voltage and tube current. The X-ray tube current is stabilized by a control circuit which adjusts the X-ray tube filament current in response to changes in the current in the X-ray tube circuit. The X-ray tube energizing voltage is stabilized by subtracting a voltage from the rectified voltage supplied to the X-ray tube. The voltage subtracted is adjustable and is increased or decreased from a nominal value to adjust for increases or decreases in the DC voltage supplied to the X-ray tube. The subtracting voltage maintains the voltage supplied to the X-ray tube at a preselected level. With the present system, therefore, much of the voltage stabilization is provided on the secondary side of the high voltage transformer.

The kilovoltage adjustment by the subtracting voltage is provided between certain limits above and below a nominal voltage value. Supplied kilovoltage adjustments greater than the adjustment range to be provided by the subtracting voltage are accomplished by adjusting the voltage supplied to the primary side of the high voltage transformer. A voltage correction control circuit is provided to determine when the subtracting voltage increases or decreases beyond its adjustment range and then adjusts the voltage on the primary side of the high voltage transformer until the secondary voltage is within the adjustment range of the subtracting voltage.

In the present system, elimination of the ripple in the energizing voltage is also accomplished by adjustment of the subtracting voltage. In the present arrangement, a high voltage capacitor is provided to smooth out the pulsating rectified voltage. An AC feedback circuit is provided to sense the remaining ripple in the rectified and filtered voltage and to cause the subtracting voltage to vary in opposite and equal relation to such ripple so that the energizing voltage appearing across the X-ray tube is substantially ripple free.

Accordingly, an object of the present invention is to provide a new and improved system for substantially eliminating the ripple in a rectified alternating current voltage.

Another object of the present invention is to provide a new and improved X-ray tube voltage supply system for substantially eliminating ripple in a high voltage supplied by a high voltage transformer-rectifier circuit to an X-ray tube.

Still another object of the present invention is to provide a new and improved system for supplying a .preselected kilovoltage to an X-ray tube and for maintaining the supplied kilovoltage at a preselected value.

Yet another object of the present invention is to provide a new and improved X-ray system wherein a voltage is subtracted from a supplied X-ray tube voltage and is controlled to eifectively pulsate in an opposite and equal relation to any ripple present in the supply voltage to substantially eliminate that ripple.

A further object of the present invention is to provide a new and improved X-ray system wherein a voltage is subtracted from a supplied X-ray tube voltage and is controlled to effectively pulsate in an opposite and equal relation to any ripple present in the rectified supply voltage to substantially eliminate that ripple and the sub; tracted voltage is further controlled to compensate for changes in the level of the supplied high voltage to maintain a constant high voltage to an X-ray tube.

A still further object of the present invention is to provide a new and improved system for supplying half-wave rectified voltage to each of a plurality of X-ray tubes wherein the supplied half-wave rectified voltage is filtered to reduce the ripple factor and the ripple is subtracted to substantially eliminate the ripple and to provide a substantially pure DC. voltage to each of the X-ray tubes.

Yet another object of the present invention is to provide an X-ray system for providing half-wave rectified voltage to each of a plurality of X-ray tubes wherein the supplied half-wave rectified voltages are substantially pure D.C. voltages and the DC. voltages are controlled to maintain them at preselected levels.

Other objects and a fuller understanding of the invention may be had by referring to the following description and claims taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a schematic circuit diagram of the present X-ray system for providing full wave rectified energizing voltage to an X-ray tube.

FIGURE 2 is a schematic circuit diagram showing the details of an X-ray contactor coil control circuit and an impedance tube safety circuit used in the present X-ray system.

FIGURE 3 is a schematic circuit diagram of the details of a kilovoltage stabilizer in the present X-ray system.

FIGURE 4 is a schematic circuit diagram of the details of a kilovoltage correction control circuit used in the present X-ray tube system.

FIGURE 5 is a schematic circuit diagram of the details of a milliampere stabilizer and a milliampere safety circuit used in the present X-ray tube system.

FIGURE 6 is a schematic circuit diagram of the details of an X-ray tube current correction circuit used in the X-ray system of the present invention. 1

FIGURE 7 is a schematic diagram of the present X-ray system for supplying half-wave rectified energizing voltages simultaneously to a plurality of X-ray tubes.

FIGURE 8 is an abbreviated schematic diagram of part of the present X-ray system including part of the current correction circuit of FIGURE 6.

Throughout the remainder of the specification, reference often will be made to specific voltage values in order to explain the operation of the various circuits shown in the drawings. These voltages and other values are by way of example only and are given to facilitate a description of the present invention.

Referring to FIGURE 1 in particular, a single X-ray tube system of the present invention is designated generally by the reference character 20. The single X-ray tube system includes a pair of main supply conductors 21, 22 which are connected to a 200 volt alternating current main power supply V. The conductors 21, 22 connect the supply V to a pair of manually adjustable input voltage tap 23, 24 of an adjustable voltage source provider 25. A pair of normally open main line switch contacts 26, 27 are provided in the conductors 21, 22. The normally open switch contacts 26, 27 are operated by a coil 28 of a main supply line contactor 29. The coil 28 is connectable across the conductors 21, 22 on the power supply side of the main switch contacts 26, 27 by a control conductor 30 and a normally open, manually operated main ON- OFF switch 31.

The adjustable voltage source provider ha a coil 34 which is contacted by the input taps 23, 24 and an adjustable output voltage tap 35. The output tap 35 is connected to and driven by a shaft (indicated schematically 'by the broken line 36) which in turn is driven by an armature 37 of a reversible alternating current servo motor 38. A suitable adjustable voltage source provider is a continuously adjustable auto transformer sold under the trademark Variac.

A pair of conductors 39, 40 are connected to the adjustable voltage tap 35 and to one end of the coil 34 respectively and provide a variable A.C. power supply for a high voltage transformer 42. The voltage across the conductors 39, 40 varies from a maximum of 220 volts When the adjustable tap 35 is at the high end of the coil 34, as viewed in FIGURE 1, to substantially zero voltage when the adjustable voltage tap 35 is driven by the servo motor 38 to the lower end of the coil 34.

voltage suitable for energizing an Xray tube 43, The high voltage transformer 42 includes a primary winding 44 and a pair of secondary windings 45, 46. The primary winding 44 is connected to the conductors 39, 40 and is energized by the variable voltage that appears across the conductors 39, 40. Adjacent terminals 47, 48 of the secondary windings 45, 46 respectively, are connected to a ground point 49 via a conductor 50 to provide a grounded center tap transformer secondary system.

A terminal 51 of the secondary winding 45 is connected to a high voltage supply conductor 52 through a rectifier 53. The rectifier 53 has its anode connected to the conductor 52 and its cathode connected to the terminal 51. A terminal 54 of the secondary winding 46 is also The high voltage transformer 42 is provided for stepconnected to the high voltage supply conductor 52 via a conductor 55 and a rectifier 56. The rectifier 56 has its anode connected to the supply conductor 52. The arrangement of the rectifiers 53, 56 with the supply conductor 52 and the secondary windings 45, 46 is such as to provide full wave rectification of the kilovoltage appearing across the secondary windings 45, 46 with the supply conductor 52 at a negative potential with respect to the ground point 49. The step-up or turns ratio of the high voltage transformer 42 is such as to provide approximately 65 kilovolts on the conductor 52 when 220 volts is impressed across the primary winding 44.

A conductor 59 connects a high voltage capacitor 58 from the high voltage supply conductor 52 to the grounded transformer terminals 47, 48. The function of the high voltage capacitor 58 is to reduce the ripple factor of the pulsating DC output provided by the high voltage transformer-rectifier circuit by means of the filtering action of the capacitor 58 as is conventional.

The X-ray tube 43 has its cathode element connected I to the high voltage supply conductor 52 via a conductor 70 and its anode element connected to ground via a conductor 71. A filament transformer 72 has its secondary winding connected in circuit with the cathode element or filament of the X-ray tube 43 to supply energy to the filament when the primary of the transformer 72 is en ergized. A current limiting resistor 74 is interposed in the high voltage supply conductor 52 between the full wave rectifier provided by the rectifiers 53, 56 and the X-ray tube 43 to protect the X-ray tube against excessive current surges.

A step-down transformer 75 has a primary winding 76 connected across the main supply conductors 21, 22 between the main switch contacts 26, 27 and the coil 34 of the adjustable voltage provider 25. A secondary winding 77 of the transformer 75 provides an auxiliary voltage supply source for the filament transformer 72 and other parts of the X-ray system. Conductors 78, 79 connect the primary winding of the filament transformer 72 across the secondary winding 77 of the auxiliary supply source transformer 75 and in series with a variable resistor 80. The variable resistor 80 is provided for fine selection and adjustment of the current in the primary of the filament transformer 72 and, consequently, of the filament current for the X-ray tube 43.

The conductor 78 further supplies energizing voltage to forward and reverse windings 83, 84 of the servo motor 38. The windings 83, 84 are jointly connected at one of their ends to a ground point 85. A phase splitting capacitor 86 is connected between the other ends of the windings 83, 84. The other end of the forward winding 83 is also connectable to the auxiliary voltage source provided by the secondary winding 77 via a conductor 88, a normally open contact 89 of a voltage increasing relay 90, a conductor 91, a normally closed contact 93 of a voltage decreasing relay 92 and the conductor 78. The other end of the reverse winding 84 is also connectable to the secondary winding 77 via a conductor 94, a normally closed cam operated low limit switch 95, a conductor 96, a normally open contact of the voltage decreasing'relay 92, and the conductor 78.

Energization of the coil 97 of the voltage increasing relay 90 closes the contact 89 to energize the forward rotation winding 83. The energized winding 83 induces forward rotation of the armature 37 to move the variable voltage tap 35 along the coil 34 in an increasing voltage direction to increase the voltage supplied to the primary 44 of the high voltage transformer 42. Energization of a coil 98 of the voltage decreasing relay 92 closes the normally open switch 100 to connect the conductor 78 to the conductor 96 thereby energizing the reverse rotation winding 84. The energized winding 84 induces reverse rotation to move the variable tap 35 along the coil 34 in a decreasing voltage direction to decrease the voltage supplied to the primary winding 44. The low limit switch 95 is cam operated (indicated schematically by the broken line 99) by the shaft 36 when the voltage tap is near its zero voltage position and opens to deenergize the motor winding 84 to thereby prevent over-running by the variable tap 35.

An X-ray contactor 102 is provided and includes an operating coil 103 and two sets of normally open switch contacts 104, 105 which are closed by the coil 103 when the coil 103 is suitably energized. The normally open switch contact 104 is interposed in the conductor 22 between the auxiliary supply transformer 75 and the adjustable voltage provider 25. The normally open switch contact 105' is interposed in the conductor 78 .between the auxiliary supply transformer 77 and the filament transformer 72. The coil 103 is connected at one end to a negative 30 volt supply. The other end of the coil 103 is connectable via a conductor 112 to a ground point 107 by an X-ray contactor coil control circuit 108. The circuit 108 is controlled principally by ON and OFF switches 109, 110. Closure ofthe ON switch 109 causes the control circuit 108 to connect the other end of the coil 103 to the ground point 107 to complete the energization circuit for the coil 103. Closure of the OFF switch 110 causes the control circuit 108 to disconnect the coil 103 from the ground point 107.

A suitable timer control circuit 111 is provided for causing the contactor coil control circuit 108 to disconnect the coil 103 from the ground point 107 at the end of a selected X-ray exposure period. Disconnection of the coil 103 from the ground point 107 by the X-ray contactor coil control circuit 108 thereby de-energizes the coil 103 and opens its contacts 104, 105 to de-energize the X-ray supply circuits.

X-ray contactor coil control circuit 108 The details of the X-ray c-ontactor coil control circuit 108 are shown in FIGURE 2. The control circuit 108 includes PNP transistors Q1, Q2 connected in a bi-stable multivibrator circuit arrangement commonly referred to as a flip-flop circuit. When the DC power supplies shown are applied, the transistor Q2 is suitably forward biased into saturation and its emitter-collector circuit is effectively conducting. At this point, the transistor Q1 is reverse biased to a point of cut-off and its emitter-collector circuit is effectively non-conducting. Closure of the ON switch 109 applies a negative trigger voltage to the base of the transistor Q1 to forward bias it and to drive it out of cut-off toward a state of conduction.

As is conventional in the flip-flop circuits, the collector current in the transistor Q1 rises and causes the collector voltage to fall. A downward change in the collector voltage is coupled to the base of the transistor Q2 and reduces its forward bias. The selector current of the transistor Q2 decreases and its collector voltage goes more negative further forward biasing the transistor Q1 and increasing conduction of the transistor Q1. This regenerative feedback continues until the transistor Q1 is in saturation and the transistor Q2 is cut-off. When the transistor Q1 is fully conducting, the conductor 112 is effectively connected to the ground point 107 and the coil 103 is energized to close the contacts 104, to supply power to the various circuits of the X-ray system.

Closure of the off switch applies a negative voltage to the base of a PNP transistor Q3, forward biasing the transistor Q3 to conduct. The conducting emitter-collector circuit of the transistor Q3 in turn applies a negative trigger voltage to the base of the transistor Q2 causing it to conduct and reducing the forward bias on the transistor Q1 to provide the regenerative feedback which continues until the transistor Q2 is in saturation and the transistor Q1 is cut-off. The energizing circuit for the coil 103 is thus effectively broken and the coil is de-energized to open the contacts 104, 105 to de-energize the X-ray circuits. The negative trigger voltage is also provided to the base of the transistor Q3 by suitable circuitry (not shown) in the timer circuit 111 to end the X-ray exposure after a selected exposure period. As will be disclosed below, negative trigger voltages are also provided to the base of the transistor Q3 via a conductor 115 from various safety circuits 116-118 in the X-ray system to also shutoff the X-ray system. Finally, a negative trigger pulse is provided to the base of the transistor Q3 by closure of a normally open high limit switch 101. The high limit switch 101 is cam operated (indicated schematically by the broken line 113) by the shaft 36 when the movable voltage tap 35 approaches the high voltage end of the winding 34.

Variable impedance tube circuit Referring to FIGURE 1, a high power regulator tube is provided and includes a plate element 126, a cathode element 127, and a control element 128. The regulator tube 125 has a high wattage plate dissipation and is preferably a tetrode having its control grid and screen grid connected together to provide the control element 128. The regulator tube 125 is interposed in the grounding conductor 50 with its plate 126 connected to the secondary winding terminals 47, 48 and its cathode element connected to the ground point 49. The regulator tube 125 is effectively a variable impedance in the grounding conductor 52 and has a nominal voltage drop which is in opposition to the voltage across the secondary windings 45, 46. In other words, the voltage across the regulator tube 125 is opposite in polarity to the rectified high voltage output provided by the secondary windings 45, 46 and the rectifiers 53, 56 so that the regulator tube 125 effectively subtracts a part of the high voltage output of the transformer 42. For example, for a 65 kilovoltage supply voltage appearing across the secondary windings, the impedance of the regulator tube 125 is set to provide a subtracting voltage drop across the tube of approximately 5 kilovolts so that 60 kilovolts is supplied to the X-ray tube 43 via the high voltage supply conductor 52. The bias applied to the control element 128 is determinative of the magnitude of the subtracting voltage across the regulator tube 125 and is varied to selectively increase or decrease the nominal 5 kilovolts subtracting voltage.

The regulator tube 125 has its control element 128 connected through a capacitor 129 and a resistor 130 to the high voltage supply conductor 52 between the current limiting resistor 74 and the X-ray tube 43 via a feedback circuit conductor 131. The capacitor 129, the resistor 130, and the conductor 131 provide an AC feedback circuit for suitably biasing the control grid of the impedance tube 125 to eliminate substantially the entire remaining peak-to-peak ripple present in the high voltage output applied to the X-ray tube 43 after the filtering action of the high voltage capacitor 58.

The filtering action of the high voltage capacitor 58 changes the pulsating DC output from the rectifiers 53, 56 to a DC voltage having a peak-to-peak ripple which is normally slightly less than 10% of the average value of the DC output. For example, a DC output of 5S kv. has

7 a peak-to-peak ripple of approximately kv. This peakto-peak ripple is passed by the AC feedback capacitor 129 and is sufficiently limited by the resistor 139 to suitably bias the impedance tube 125 to provide a pulsating subtracting voltage which opposes this kv. peak-to-peak ripple and substantially eliminates that ripple. The subtracting voltage across the impedance tube is thus a pulsating voltage which is substantially equal to and opposes the ripple voltage to thereby substantially eliminate the ripple. By subtracting the ripple, an almost pure DC voltage is supplied to the X-ray tube 43. With the present system, the ripple is reduced to a maximum of 50 volts for as high as 60 kilovolts supplied to the X-ray tube.

Kilovoltage selection circuit 135 A kilovoltage selection circuit 135 is provided for sensing the kilovoltage supplied across the X-ray tube 43 and for selectively adjusting the supplied kilovoltage, FIG- URE 1. The kilovoltage sensing and selecting circuit 135 includes a high impedance resistor 136, a kilovoltage meter 137, a resistor 138, and a kilovoltage selector 139 connected between a ground point 140 and the cathode of the X-ray tube 43 via a conductor 1-42 and the conductor 52. The resistor 136 is of a very high impedance to prevent significant drain-off of current from the X-ray tube circuit. One suitable resistance value for the resistor 136 has been found to be 50 megohms. The kilovoltage selector 139 is effectively a variable resistor and has a plurality of kilovoltage adjustment positions at approximately 2 kv. intervals from 12 kv. to 60 kv. The voltage level of a voltage sensed across the selector resistor 139 is introduced into the input of a kilovoltage stabilizer control circuit 145 via a conductor 146.

Kilovoltage stabilizer control circuit 145 Referring to FIGURE 3, the kilovoltage stabilizer control circuit 145 includes transistors Q4-Q7 arranged in what is essentially a differential or difference amplifier circuit. A fixed reference signal of, for example, a negative 9 volts is applied to the base of the NPN transistor Q4. When the voltage on the high voltage supply conductor 52 is the voltage value selected on the kilovoltage selector resistor 139, then a negative 9 volts drop is across the resistor 139. The negative 9 volts sensed across the selector resistor 139 by the conductor 146 is applied to the base of the NPN transistor Q5 and is compared against the negative 9 volts fixed reference signal applied to the base of the transistor Q4. As long as the kilovoltage on the high voltage supply conductor 52 is that which has been selected by the selector-resistor 139, there is no difference between the sensed voltage signal and the fixed reference signal and the stabilizer circuit 145 is effectively inactive. If the kilovoltage on the high voltage supply conductor 52 should undergo an unselected increase or decrease, for example, because of an undesired load or line condition, then the sensed voltage signal momentarily decreases or increases proportionally to such a kilovoltage change. The differential amplifier in the stabilizer circuit compares the increase or decrease in the sensed voltage signal to the fixed reference voltage and the difference is amplified and applied to the base of the PNP transistor Q7. The collector of the transistor Q7 is connected to the control element 128 of the regulator tube 125 via a difference output conductor 147. The difference between the sensed voltage signal and the fixed reference signal, the sensed voltage signal being either greater or smaller, causes the transistor Q7 to drive the control 128 of the regulator tube 125 either less or more positively to either raise or lower the subtracting voltage provided across the tube which consequently either lowers or raises the kilovoltage supplied to the X-ray tube thereby maintaining a predetermined kilo voltage across the X-ray tube. For example, if the kilovoltage to be supplied to the X-ray tube is set at 30 kv. on the kv. selector 139 and due to a line condition suddenly goes to 32 kv. then the sensed voltage signal increases a proportional amount and differs from the reference voltage by that amount. This difference is applied to the base of the transistor Q7 which then drives the control element 128 of the regulator tube less positively to increase the subtracting voltage across the regulator tube 125 by two kilovolts. The two kv. increase additionally opposes the rectified output of the high voltage transformer and lowers the kilovoltage supplied to the X-ray tube to 30 kv. If the supplied kilovoltage should decrease from the selected value, then the control element 128 of the regulator tube is driven more positively to decrease the voltage across the tube which then raises the supplied kilovoltage to the selected level. The nominal subtracting voltage (5 kilovolts as used in the example herein) of the regulator tube 125 is thus increased or decreased by the kilovoltage stabilizer circuit in response to the sensed voltage to lower or raise the kilovoltage supplied to the X-ray tube to maintain a stable constant kilovoltage to the X-ray tube. This is accomplished by the regulator tube 125 while it also supplies the pulsating kilovoltage to eliminate the ripple in the kilovoltage supplied to the X-ray tube 43.

In the example used herein, the range of adjustment provided by the regulator tube is 2.5 kilovolts on either side of the nominal 5 kv. subtracting voltage across the regulator tube. In other words, the range of adjustment of the regulator tube voltage is from 2.5 kv. to 7.5 kv. with the tube initially at a 5 kv. midpoint. During operation of the X-ray system, the regulator tube voltage automatically adjusts to compensate for X-ray tube voltage changes and will usually be somewhere within the 2.5 kv.- 7.5 kv. range so that more or less than a 2.5 kv. adjustment may be available depending upon the regulator tube voltage at the time and whether the regulator tube voltage must increase or decrease.

If the regulator tu'be voltage is at the 5 kv. nominal voltage level then a 2.5 kv. adjustment in either direction is available. If the unselected change in kilovoltage is more than the kv. adjustment available from the regulator tube so that the control element 128 is biased to drive the tube voltage beyond its 2.5 kv. to 7.5 kv. adjustment range, than a major kilovoltage adjustment control circuit is utilized to cause the voltage provider drive motor 38 to position the movable voltage tab 35'to raise or lower the supply voltage until the supplied kilovoltage is again within the adjustment range of the regulator tube 125.

Major kilovoltage adjustment control circuit 150 Referring to FIGURE 4, the major kilovoltage adjustment control circuit 150 is effectively a dead band amplifier which causes energization of one or the other of the windings 97, 98 only if the kilovoltage across the regulator tube 125 goes below or above certain fixed limits, for example, 2.5 kv. and 7.5 kv. which is the adjustment range provided by the regulator tube 125. As long as the kilovoltage across the regulator tube 125 is within the fixed limits of 2.5-7.5 kv., the kilovoltage adjustment control circuit 150 is effectively inactive or dead for a "band zone of 2.5 to 7.5 kv.

The kv. adjustment control circuit 150 includes a regulator tube voltage sensing input conductor 151 and a pair of coil energizing output conductors 152, 153. The input conductor 151 is connected to the plate 126 of the regulator tube 125 and applies a current signal through a high impedance limiting resistor to the kilovoltage adjustment control circuit 150 which current signal is representative of the kilovoltage across the regulator tube 125. The output conductors 152, 153 are connected to the voltage increasing and voltage decreasing relay coils 97, 98 to energize one of the coils 97, 98 when the respective one of the conductors 152, 153 is effectively connected to the ground point 154. The limiting impedance resistor 155 is a very high impedance on the order of five megohms to limit the current drawn from the conductor 50 in the X-ray tube circuit.

The kv. adjustment control circuit 150 includes trans stors Q8-Q21 arranged in four distinct transistor circuits which co-operate to make up a dead band amplifier. The four transistor circuits include the transistors Q8-Q13 arranged as a difierential amplifier 158 for directlycontrolling energization of the coils 97, 98; PNP transistors Q14, Q15 arranged as a flip-flop circuit to provide a grounding circuit 159; NPN transistors Q16 and Q17 arranged as a difierential amplifier 160 for turning on the grounding circuit 159 when the regulator tube voltage goes below or above the head band zone of 2.5 kv. to 7.5 kv.; and NPN transistors Q18 and Q19 arranged as a differential amplifier 161 for turning off the grounding circuit 159 when the regulator tube voltage is substantially at its nominal kilovolt value.

In the difierential amplifier 158, the bases of the transistors Q and Q13 are connected to a negative 18 volt fixed reference via resistors 162, 163 respectively. The base of the transistor Q10 is also connected through a resistor 164 and the emitter-collector circuit of a grounded base transistor Q20 to a positive 18 volt supply via a conductor 165. The resistor 164 is sized such that approximately one milliamp flows in the conductor 165 and through the resist-or 162 to provide a fixed bias of a negative 10 volts on the base of the transistor Q10. This negative 10 volt bias on the base of the transistor Q10 is effectively reflected to the base of the PNP transistor Q8.

When the 5 kilovolt nominal voltage is across the regulator tube, a current of approximately one milliamp flows in the conductor 151 through the emitter-collector circuit of grounded base transistor Q21 to the base of the transistor Q13 and through the resistor 163. The resistor 163 is sized to provide a bias of a negative 10 volts on the base of the transistor Q13 when the voltage drop across the regulator tube 125 is 5 kilovolts. This negative 10 volt bias on the base of the transistor Q13 is effectively reflected to the base of the transistor Q11. The bias on the transistors Q13 and Q11 is thus directly dependent on the kilovoltage across the regulator tube 125 and is a negative 10 volts when 5 kilovolts is across the regulator tube 125. The negative 10 volt bias on the bases of the transistors Q10 and Q8 is a constant or fixed reference against which the voltage on the bases of the transistors Q13 and Q11 may be compared.

The emitter-collector circuits of the transistors Q11 and Q8 are both normally in a somewhat conductive state which may be described as half-on. The current passed by the transistors Q8, Q11 in this state is not suflicient to energize either of the coils 97, 98 sufliciently for them to operate their respective switch contacts. As the kilovoltage across the regulator tube 125 increases or decreases, the negative bias on the bases of the transistors Q13 and Q11 decreases or increases respectively. A decrease in the bias on the base of transistor Q11 renders it less conductive and the transistor Q8 more conductive. An increase in the bias on the base of the transistor Q11 has the opposite effect on the transistor Q8.

The emitters of the transistors Q11 and Q8 are connected through a resistor 167 to the collector of the transistor Q15 in the flip-flop grounding circuit 159 via a conductor 168. The emitters of the transistors Q14, Q15 in the flip-flop grounding circuit 159 are connected to the ground point 154. As shown in the drawing, the arrangement of the transistors Q14, Q15 in the flip-flop grounding circuit 159 is such that when the positive 18 volts and the negative 30 volts supplies are applied, the transistor Q14 is fully ON and the transistor Q15 is cut off. When the transistor Q15 is cut off, sufiicient negative voltage is applied to the emitters of the transistors Q11, Q8 in the differential amplifier 158 to render the emitters sufficiently negative with respect to the bases to prevent either transistor Q11 or transistor Q8 from going to a state of full conduction regardless of the change in voltage at the base 10 of the transistor Q11 due to kilovoltage changes ac the regulator tube 125. To permit one of the transistors Q11, Q8 to turn full ON and the other to turn full OFF, it is necessary that the emitters of the transistors Q11, Q8 be near ground potential.

The emitters of the transistors Q11, Q8 are placed near ground potential only when the flip-flop grounding circuit 159 is switched or flipped to its opposite stable state where the transistor Q15 is full ON and the transistor Q14 is cut off by applying a negative voltage signal to the base of the transistor Q15. When the transistor Q15 is full ON, it connects the conductor 168 to the ground point 154 to place the emitters of the transistors Q8, Q11 near ground potential. This negative trigger pulse is provided by the zone determining differential amplifier via conductors 169, 170 when the diiferential amplifier 160 determines that the voltage across the regulator tube is beyond the 2.5-7.5 kilovolt adjustment range provided by the regulator tube 125.

In the zone determining differential amplifier 160, a conductor 171 supplies the negative 10 volt fixed reference from the base of the PNP transistor Q8 to one side of a resistor 172 having its other side connected to the base of the NPN transistor Q16. A conductor 173 connects the emitter of the NPN transistor Q17 to the constant negative 10 volts reference through a Zener diode 174 to provide a constant reference of approximately 7.7 volts to the emitter of the transistor Q17. A conductor 175 supplies the variable voltage signal at the base of the transistor Q11 to one side of a resistor 176 having its other side connected to the base of the transistor Q17. A conduct-or 177 connects this same variable voltage signal to the emitter of the transistor Q16. A conductor 178 connects together the collectors of the NPN transistors Q16, Q17 and includes diodes 179, having their cathodes connected to the collectors of transistors Q16, Q17, respectively. The conductor 1-69 is connected to the anodes of diodes 179, 180 to transmit a negative pulse to the base of the transistor Q15 in the flip-flop grounding circuit 159 whenever either of the transistors Q16, Q17 conducts.

Initially, neither of the transistors Q16, Q17 is in a state of conduction. If the representative regulator tube voltage at the base of the transistor Q11 goes more nega tively, then the transistor Q16 will conduct when its emitter is sufiiciently more negative than its base. When the transistor Q16 conducts, it sends a negative pulse through the diode 179 to the base of the transistor Q15 in the flipflop grounding circuit via conductors 1 69, 177. If the representative regulating tube voltage at the base of the transistor Q11 goes more positively (i.e., less negatively) then the transistor Q17 will conduct when its base is sufficiently more positive than its emitter to send a negative trigger pulse to the base of the transistor Q15 in the flipflop circuit 159 to cause the flip-flop grounding circuit 159 to switch to its other stable state wherein the transistor Q15 is conducting.

The arrangement of the transistors Q16 and Q17 in the differential amplifier 160 is such that the transistor Q16 conducts to send a negative trigger pulse to the flip-flop grounding circuit 159 when the voltage across the regulatmg tube 125 reaches its lower limit, for example, 2.5 kv., and the transistor 17 conducts to send a negative trigger pulse to the flip-flop grounding circuit 159 when the voltage across the regulating tube 125 reaches its upper limit, for example, 7.5 kv. The 2.5 kv. lower limit is the nominal 5 kv. regulator tube voltage less the 2.5 kv. adjustment provided by the regulator tube and the 7.5 kv. limit is the 5 kv. nominal voltage plus the 2.5 adjustment provided by the regulator tube. The differential amplifier 160 is thus effectively inactive for a zone of approximately 5 kv. plus or minus 2.5 kv. which corresponds to the range of kilovoltage adjustment provided by the regulator tube 125.

The arrangement of the transistors Q18 and Q19 in the differential amplifier 161 is very similar to the arrangement of the transistors Q16, Q17 in the differential amplifier 160. A conductor 182 connects the cathodes of diodes 183, 184 to the collectors of the transistors Q18, Q19 respectively, and the anodes of the diodes 183, 184 to the base of the transistor Q15 via a conductor 185 and the conductor 170. When the representative regulator tube voltage signal on the base of the transistor Q11 goes more negative, the transistor Q18 conducts. When the representative regulating tube voltage signal on the base of the transistor Q11 goes less negative, then the transistor Q19 conducts. The transistors Q18 and Q19 turn on at approximately the same voltage points as do the transistors Q16, Q17 respectively. However, the negative trigger pulses produced by either of the transistors Q18 or Q19 is blocked by the diode 186 in the conductor 185 from reaching the base of the transistor Q15 in the flip-flop grounding circuit 159. Thus, it is only the differential amplifier 160 which turns on the flip-flop grounding circuit 159 to permit either transistor Q11 or Q8 to fully conduct and energize the coils 97, 98 respectively and initiate operation of the voltage provider drive motor 38 to increase or decrease the supply voltage as may be required.

When the movable voltage tab 35 of the adjustable voltage provider 25 has been adjusted by the drive motor 38 to increase or decrease the kilovoltage across the regulating tube 125 as is required and the voltage across the regulating tube 125 approaches its nominal value of, for example, kilovolts, the negative emitter-base bias of the conducting one of the transistors Q16, Q17 decreases such that the conducting transistor cuts off and a positive voltage appears at the juncture of conductors 169, 178. This happens when the regulating tube voltage is again within the 2.5-7.5 kv. dead zone but is not quite to the 5 kv. nominal voltage of the regulating tube. A diode 188 in the conductor 169 blocks the positive voltage signal from the flip-flop grounding circuit input conductor 170 and thus prevents the differential amplifier 160 from flopping the grounding circuit 159 back to its initial stable state wherein the transistor Q15 is cut oil.

In the differential amplifier 161, as the regulator tube kilovoltage is brought back within the 2.57.5 kilovolt dead zone, the conducting one of the transistors Q18, Q19 does not shut oil? until the negative emitter-to-base bias is substantially zero which occurs when the representative voltage at the base of transistor Q11 is within the 0.3 to 0.4 volt of the fixed reference voltage signal.

The 0.3 to 0.4 volt being the emitter to base voltage drop of the transistors Q18, Q19.

Thus, the conducting one of the transistors Q18, Q19 does not turn off until the representative voltage signal at the base of the transistor Q11 is volts plus or minus 0.3 to 0.4 volt, which corresponds to the nominal 5 kilovolts across the regulator tube 125 plus or minus 250 volts. When the latter condition is reached, the conducting one of the transistors Q18, Q19 cuts off so that a positive voltage appears at the juncture of the conductors 182, 185. This positive voltage is conveyed by the conductor 185 through the diode 186 to the input conductor 170 of the flip-flop grounding circuit 159 and flops the flip-flop 159 back to its initial stable state where the transistor Q is cut-off to de-energize the energized one of the relay coils 97, 98. Thus, the dilferential amplifier 160 only turns on the flip-flop grounding circuit 159 when the kilovoltage across the regulating tube goes beyond the dead zone of 2.5 to 7.5 kilovolts and the diiferential amplifier 161 only turns off the flip-flop grounding circuit 159 when the kilovoltage across the regulator tube 125 is again within approximately 250 volts of its nominal 5 kilovolt value.

A conductor 190 connects the collector of the transistor Q2 in the X-ray contactor coil control circuit 108 to the base of the transistor Q13 in the major kilovoltage adjustment control circuit 150. Whenever a negative trigger pulse is applied to the base of the transistor Q3 to flop the flip-flop circuit of the X-ray contactor coil control circuit 108 back to its initial stable state so that the transistor Q1 is cut off and the transistor Q2 is full on, a positive voltage signal is applied through the properly poled diode 191 to the base of the transistor Q13. This positive voltage signal drives the base of the transistor Q13 more positively which is reflected to the base of the transistor Q11 driving it more positively. The major kilovoltage adjustment control circuit then operates, as has been explained above, to turn the transistor Q11 full off and the transistor Q8 full on to energize the voltage decreasing relay coil 98 to close the contact switch 93. Closure of the contact switch 93 energizes the motor winding 84 which induces rotation of the armature 37 and the shaft 36 to move the variable tap 35 in a decreasing voltage direction. The winding 84 will remain energized until the variable tap 35 is in its zero voltage position. Thus, each time the X-ray tube system is turned off by applying a negative voltage pulse to the base of the transistor Q3, the adjustable voltage provider drive system is energized to move the adjustable voltage tap 35 to a zero voltage position.

X-ray tube current selection Referring again to FIGURE 1, a variable resistor 196 is interposed in the grounding conductor 50 between the regulator tube 125 and the ground point 49. The variable resistor 196 functions as a current selector for the X-ray tube 43. The variable resistor 196 is in series in the X-ray tube circuit and its resistance value setting is selective of the current in the X-ray tube circuit. The selector arm of the variable resistor 196 has a plurality of milliamperage (ma) selection positions for selecting the desired X-ray tube current between approximately 2 ma. and 50 ma. for the single X-ray tube system shown. A milliameter 197 is provided in series in the conductor 50 between the regulator tube 125 and the ma. selector resistor 196.

Ma. stabilizer control circuit 200 A ma. stabilizer control circuit 200 is provided to maintain a constant selected current in the X-ray tube circuit as selected by the ma. selector 196. The ma. stabilizer control circuit 200 has an input conductor 202 connected to the grounding conductor 50 between the milliameter 197 and the resistor 196 to provide an input voltage signal which is proportional to and representative of the current in the X-ray tube circuit. The ma. stabilizer 200 further has an output conductor 203 connected to the primary of a transformer 204. The secondary of the transformer 204 is connected in series in the filament'supply circuit between the ma. current fine adjusting resistor 80 and a ground point 205. The transformer 204 provides effectively a variable impedance in the filament supply circuit to provide automatic adjustment of the current in the supply circuit as is required to stabilize the tube current at a selected value.

Referring to FIGURE 5, the milliameter stabilizer circuit 200 includes transistors Q23-Q27 arranged as a differential amplifier 201. The base of the NPN transistor Q27 is connected to the conductor 202 so that the voltage across the ma. selector resistor 196 is applied to the base of the transistor Q27. If the current in the X-ray tube circuit is that selected by the particular setting of the ma. selector resistor 196, then the voltage across the resistor 196 will have a predetermined value, for example, a positive 9 volts. The base of NPN transistor Q26 is connected to a fixed voltage reference point 207 on a conductor 208. The voltage at the reference point 207 is fixed also at a positive 9 volts value by a Zener diode 209 having its anode connected to a ground point 210 and its cathode connected by a conductor 208 through a resistance 211 to a positive 30 volts supply.

As long as the current is stable in the X-ray tube circuit, the representative voltage sensed across the ma.

change in the X-ray tube circuit, either increase or decrease, increases or decreases the representative voltage sensed across the ma. selector resistor 196 so that the sensed voltage at the base of the transistor Q27 differs from the reference voltage signal at the base of the transistor Q26. This difference is amplified and applied to the transistor Q23 to change its emitter to base bias voltage so that current conduction in the transistor Q23 increases or decreases to increase or decrease the bias voltage applied. to the base of the PNP transistor Q28. A change in the bias at the base of the transistor Q28 causes current to pass more or less easily in a variable impedance circuit 215 formed 'by PNP transistors Q29, Q30 and causes a change of impedance in the impedance circuit 215. The change of impedance in the impedance circuit 215 is ettectivel reflected to the secondary of the impedance transformer 204. The change in the impedance of the impedance tranformer 204 is such as to change the voltage at the primary of the filament transformer 72 to adjust the X-ray tube current back to the value originally selected by the ma. selector resistor 196.

To illustrate the operation of the ma. stabilizer circuit, assume that for some reason, for example a load change, there is an unselected increase in the current in the X-ray tube circuit. This increase in current causes an increase in the representative voltage across the ma. selector resistor 196 thereby increasing the voltage at the base of the transistor Q27 and providing a voltage diiterence when the representative voltage is compared against the 9 volt positive voltage reference at reference point 207. This difference is amplified and applied to the base of the transistor Q28 which, through the variable impedance circuit 215, eifectively increases the impedance across the secondary Winding of the transformer 204. The increase in the impedance of the transformer 204 causes less voltage to be supplied to the primary of the filament transformer 72 thereby decreasing the filament current and consequently the X-ray tube circuit current until it is at the value selected on selector resistor 196.

Milliampere correction circuit 217 The ma. correction circuit 217 is provided to assure that true X-ray tube current is sensed by the ma. stabilizer 200 and is measured by the milliameter 197. The actual current Ia in the conductor 50 is substantially equal to the actual X-ray tu'becurrent It in the conductor 70 plus the current, 1b in theconductor 142 of the X-ray tube voltage sensing circuit 135 less the current in the conductor 151which latter current provides an indication of the regulator tube voltage. .This, current relationship is shown by the schematic diagram in FIGURE 8 of part of the X-ray tube system. Represented in equation form:

The currents lb, 10 in the conductors 142, 151 are largely determined by the extremely high resistance values of the resistors 136 and 155 since the values of the other resistances in the circuits are extremly small in comparison. The ma. correction circuit effectively subtracts the X-ray tube voltage sensing circuit current Ib and adds the regulator tube voltage sensing circuit current 10 to the current Ia in the regulator tube circuit conductor 50 prior to the ma. meter 197 and the ma. selector resistor 196 so that only the actual X-ray tube current It is in the ma. selector resistor 196.

Referring to FIGURE 6, the ma. correction circuit 217 includes a current correcting conductor 218 connected to the conductor 50 between the regulator tube 125 and the ma. meter 197, a conductor 219 connected to the X-ray tube voltage sensing circuit conductor 142 between the kilovoltage meter 137 and the resistance 138, and a conductor 220 connected via the conductor 175 to the base of the transistor Q11 in the major kilovoltage adjustment control circuit 150. A resistor 221 is provided in the con- 14 ductor 218 between the conductors 175 and 50 so that a current Ir is in the conductor 218.

A fixed current providing circuit 222 is connected to the conductor 218 between the conductor 50 and the resistor 221 to add a fixed current If to the currents in the conductor 50. The fixed current providing circuit 222 includes a pair of resistors 223, 224 connected in series between the conductor 218 and a positive 30 volt supply, and a Zener diode 225 having its anode connected to a ground point 226 and its cathode connected to the adjacent ends of resistors 223, 224. The Zener diode 225 is sized so that approximately 18 volts positive appears across the Zener diode 225. The circuit 222 supplies the fixed current If to the conductor 218 regardless of the other currents and conditions in the system.

An X-ray tube voltage sensing circuit current withdrawing circuit 227 is connected to the conductor 219 and to the conductor 218 between the resistor 221 and the fixed current providing circuit 222. The current withdrawing circuit 227 includes a PNP transistor Q31 having its collector connected to a negative 30' volt supply, its emitter connected through a resistor 228 to the conductor 218 and its base connected to an emitter of a NPN transistor Q32. The transistor Q32 has its collector connected to the ground point 229 and its base connected to the conductor 219.

The transistors Q31, Q32 efiectively connect the end of the resistor 228 to the negative voltage side of the resistor 138. The voltage across the resistor 228 is the diflerence between the positive nine volts provided by the ma. selector resistor 196 and the voltage at a voltage point 238 on the conductor 142. The voltage at the voltage point 238 is equal to the voltage across the selector resistor 139 and the resistor 138. The voltage across the selector resistor 139 is maintained at a negative nine volts by the kv. stabilizer 145. The voltage across the resistor 138 is dependent upon the current 112 in the conductor 142. For example, when the conductor 52 is at 50 kv. there is one milliampere in the conductor 142 because of the 50 megohm resistor 136. The resistor 138 is sized to have approximately a ten volt drop with 50 kv. on the conductor 52. A change in the current Ib in the conductor 142 causes a corresponding change in the voltage across resistor 138.

A current Id is present in the resistor 228 because of the voltage across the resistor 228 and is effectively subtracted from the current Ia in the conductor 50. The current Id is the sum of the current Ib' and an error current Ie. The current Ib is equal to the current Ib in conductor 142. The error current Ie is a fixed current present in the resistor 228 because of the negative nine volts provided by selector resistor 139 which is a constant factor in the voltage applied to the resistor 228. Explained another way, if the resistor 138 were referenced to ground potential instead of to a negative nine volts, then the voltage applied to the resistor 228 would be caused solely by the current lb in the conductor 142. However, since the resistor 138 is referenced to a negative nine volts, then there will be current Ie in the resistor 228 which is caused by the negative nine volts reference for the resistor 138, and a current 112 which is made equal to 112 by suitably sizing the components of the current withdrawing circuit 227.

The current Ir in the resistor 221 is dependent upon the negative voltage at the base of transistor Q11 in the major kilovoltage adjustment control circuit Changes in the negative voltage at the base of the transistor Q11 produces corresponding changes in the current Ir in the resistor 221. Since changes in the negative voltage at the base of the transistor Q11 are inversely proportional to changes in the current 10 in the conductor 151, changes in the current Ir are inversely proportional to changes in the current Ic.

The arrangement and sizing of the elements of the fixed current providing circuit 222 and of the resistor 221 are such that the fixed current If provided by the current adding circuit 222 less the current Ir in the resistor 221 and 15 less the error current Is is equal to the current in the conductor 151. Expressed in equation form:

Since the conductor 218 subtracts a current Ir from the current Id, the conductor 223 adds a current I to the current Ia in the conductor 50, and the current Id is equal to the current Ib plus the error current Ie, then it follows that the current present in the milliameter 197 and in the ma. selector resistor 196 is the actual X-ray tube current It. Expressed in equation form:

Provided that The provision of a fixed current If supplied to the positive 9 volt voltage point from a fixed positive 18 volt supply enables the current in conductor 50 to be corrected for the current Ic withdrawn by the conductor 151 by using a suitable negative voltage found at the base of the transistor Q11, which negative voltage is related to the current Ic. In other words, it is not readily possible to provide a current to the positive voltage point at the end of the resistor 196 from a negative voltage point at the base of the transistor Q11. The negative voltage at the base of the transistor Q11 may, however, be used to Withdraw a current which is related to the current Ic from the conductor 50. This withdrawn current is the current Ir. By adding to the conductor 50 a current I which is substantially larger than the current Ic, and subtracting a current Ir, which is inversely proportional to the withdrawn current Ic, the current in the conductor 50 is corrected for the Withdrawn current Ic.

Kv. safety circuit Referring to FIGURE 3, the kilovoltage safety circuit 118 is provided to cause the X-ray contactor coil 103 to be de-energized to shut-down the X-ray system whenever the kilovoltage across the X-ray tube 43 exceeds certain limits. The kilovoltage safety circuit 118 includes a PNP transistor Q33 having its base connected to the negative 9 volt sensed voltage point between the resistor 138 and the kilovoltage selector resistor 139 via a conductor 231 and the conductor 146. The emitter of the transistor Q33 is connected to the anode of a Zener diode 233 via a conductor 232 which also connects the cathode of the Zener diode 233 to a negative 9 volt fixed reference voltage point at the base of the transistor Q4. The Zener diode 233 is sized to hold the emitter of the transistor Q33 at approximately a negative 11.3 volts so that the transistor Q33 is forward biased to a state of conduction only when the voltage applied at its base goes increasingly negative to approximately a negative 12 volts.

The collector of the transistor Q33 is connected to the base of a NPN transistor Q34 via a conductor 235. The emitter of the transistor Q34 is connected to a negative 30 volt supply and its collector is connected via a conductor 236 through a diode 237 to the conductor 115 of the X ray contactor coil control circuit 108.

When the kilovoltage across the X-ray tube 43 exceeds that set by the kilovoltage selector resistor 139, and the sensed voltage point intermediate between the resistors 138, 139 increases beyond its negative 9 volts normal level to approximately a negative 12 volts or more, then the transistor Q33 conducts to trigger the transistor Q34. The transistor Q34, being now in a conductive state, sends a negative 30 volt trigger pulse through the diode 237 to the base of the transistor Q3 in the X-ray contactor coil control circuit 108 via the conductors 236 and 115. The transistor Q3 then triggers the flip-flop transistors Q1, Q2,

16' to de-energize the X-ray contactor coil 103 and shut down the X-r-ay system except for the power supplied to the windings 83, 84 of the voltage provider drive motor 38.

Ma. safety circuit 117 Referring to FIGURE 5, the milliampere safety circuit 117 is provided to cause the X-ray contactor coil 103 to be de-energized to shut-down the X-ray system whenever the milliarnperage (ma.) in the X-ray tube circuit exceeds a predetermined limit. The ma. safety circuit includes a NPN transistor Q35 having its base connected to the conductor 50 between the ma. selector resistor 196 and the millia-meter 197 via a conductor 240 and the conductor 202. The emitter of the transistor Q35 is connected to the positive 9 volt reference point 207 at the base of the transistor Q26 through a Zener diode 241 via a conductor 242. The Zener diode 241 is sized to hold the emitter of the transistor Q35 at approximately a positive 11.3 voltage level. The collector of the transistor Q35 is connected to the base of a NPN transistor Q36 through a Zener diode 243. The collector of the transistor Q36 is connected to a positive 18 volt supply and its emitter is connected through a diode 244 to the base of the transistor Q3 in the X-ray contactor coil control circuit 108 via a conductor 245, the conductor 236, and the conductor 115.

Normally, the voltage across the ma. selector resistor 196 will be a positive 9 volts when the X-ray tube current is at the value selected by the ma. selector 196. If the current in the X-ray tube circuit should make an unselected increase, the voltage across the ma. selector resistor 196 increases, which increase is applied to the base of the transistor Q35. When the current in the tube circuit increases an unselected amount sufiicient to cause the voltage across the resistor 196 to increase greater than the 11.3 volts onthe emitter of the transistor Q35 then the transistor Q35 conducts to trigger the transistor Q36 which sends a negative trigger pulse to the base of the transistor Q3 in the X-ray contactor coil control circuit 108 to cause the contactor coil 103 to be de-energized and shut down the X-ray system except for the adjustable volttage provider drive motor 38.

Regulator tube safety circuit 116 Referring to FIGURE 2, the regulator tube safety circuit 116 is provided to cause the X-ray contactor coil 103 to be de-energized to shut down the X-ray system whenever the voltage across the regulator tube exceeds a predetermined limit. The regulator tube safety circuit 116 includes a NPN transistor Q37 having its base connected to the base of the transistor Q11 in the major kilovoltage adjustment control circuit via a conductor 250 and the conductor 175. The emitter of the transistor Q37 is maintained at approximately a negative 4 volts through a voltage divider network comprising Zener diodes 251, 252 having their anodes connected to a negative 30 volt supply via a conductor 253. The collector of the transistor Q37 is maintained at a positive potential provided by a positive 30 volt supply via resistors 254, 255 and a Zener diode 256.

The maximum permissible voltage for the regulator tube 125 is approximately 10 kilovolts. When 10 kilovolts appear across the regulator tube 125, the voltage at the base of the transistor Q11 in the major kilovoltage adjustment control circuit 150 goes less negative to approximately a negative 2 volts which places the base of the transistor Q37 more positive than its emitter. With the base of the transistor Q37 more positive than its emitter, the transistor Q37 conducts and sends a negative trigger pulse via diode 257 and conductor 115 to the base of the transistor Q3 in the contactor coil control circuit 108 to cause the coil 103 to be de-energized and shut down the X-ray system.

17 Summary of operation of single X-ray tube system 20 Initially, the ma. selector 196 and the kv. selector 139 are set to the desired ma. and kv. positions to provide the milliamperage and the kilovoltage required for the exposure. Closure of the switch 31 (FIGURE 1) energizes the coil 28 to close the main contacts 26, 27 thereby energizing the primary 76 of the auxiliary power supply transformer 75. Closure of the ON pushbutton 109 sends a negative trigger pulse to the base of the flip-flop transistor Q1 in the X-ray contactor coil control circuit 108 to turn on the transistor Q1 thereby energizing the Xray contactor coil 103. The energized coil 103 closes its contacts 104, 105. The closed contact 104 applies voltage to the coil 34 of the adjustable voltage provider 25. The closed contact 105 energizes the instrument transformer 72.

Since the variable tap 35 of the adjustable voltage provider 25 is in a zero position, none of the supply voltage is applied to the primary 44 of the high voltage transformer 42 and consequently no voltage is supplied to the X-ray tube. The voltage on the X-ray tube is lower than that called for by the setting on the kv. selector 139, and the sensed voltage signal appearing across the kv. selector resistor 139 is less than the negative 9 volt reference voltage provided to the differential amplifier in the kv. stabilizer 145 (FIGURE 3). This difference is suitably amplified by the differential amplifier and drives the control element 128 of the regulator tube 125 more positively via the conductors 147, 129. The kilovoltage across the regulator tube decreases from its nominal 5 kilovolt value. The difference between the kv. setting on the kv. selector 139 is more than can be compensated for by the adjustment of 2.5 kilovolts provided by the regulator tube 125, and the voltage across the regulator tube 125 decreases to below its 2.5 kilovolt lower adjustment range limit. At this point in the kv. adjustment control circuit 150, the voltage on the base of the transistor Q11 has gone less negative so that the transistor Q11 is substantially on and the transistor Q8 is substantially off. Also at this point, the dead zone determining differential amplifier 160 of the kilovoltage adjustment control circuit 150 has been initiated to send a trigger pulse to the base of the transistor Q15 in the grounding circuit 159 to turn on the transistor Q15, thereby substantially grounding the emitter of the transistor Q11 causing it to turn hard or full on and energize the voltage increasing relay coil 90. The energized relay coil 90 closes its normally open contact 89 (FIGURE 1) to energize the motor winding 83 from the secondary of the auxiliary supply transformer 75 via the conductor 88, the normally open contact 89, the conductor 91, the switch contact 93, and the conductor 78. The energized motor winding 83 induces rotation of the armature 37 .to rotate the shaft 36 to drive or position the movable tap 35 toward an increasing voltage position along the coil 34. This increases the voltage applied to the primary 44 of the high voltage transformer 32 and consequently increases the kilovoltage supplied to the X-ray tube 43 via the supply conductor 52. As the kilovoltage in the supply conductor 52 increases, the sensed voltage across the selector resistor 139 also increases. As the kilovoltage in the supply conductor 52 reaches the value selected on the kilovoltage selector resistor 139, the difference between the negative 9 volt reference applied to the base of the transistor Q4 in the kv. stabilizer 145 and the sensed voltage applied to the base of the transistor Q5 diminishes so that the kilovoltage stabilizer circuit 145 decreases the driving voltage applied to the control grid 128 of the regulator tube 125. The impedance of the tube'125 correspondingly increases and the kilovoltage drop across the regulator tube increases toward the positive 5 kv. nominal voltage drop. When the kilovoltage in the supply conductor 52 is approximately within 250 kilovolts of the kilovoltage selected by the kv. selector 139, the differential amplifier circuit 161 in the kv. adjustment control circuit 150 sends a trigger pulse to the base of the transistor Q15 in the flip-flop grounding circuit 159 to turn the transistor Q15 off thereby causing the voltage increasing relay coil 90 to be de-energized. The contact 89 of the de-energized voltage increasing relay 97 opens to deenergize the motor winding 83 thereby stopping the positioning movement of the movable voltage tap 35 of the adjustable voltage provided 25. The kilovoltage level in the supply conductor 52 is now at the value selected by the kilovoltage selector 139 and the X-ray tube is now operating at the full selected kilovoltage level.

If the X-ray exposure is to be a timed exposure, a suitable circuitry (not shown) activates the X-ray timer 111 and the X-ray timer runs for the period preselected on it. At the end of the timed exposure period, the X-ray timer 111 sends a negative trigger pulse to the base of the transistor Q3 in the X-ray contactor coil control circuit 108 to trigger the flip-flop in the control circuit 108, and turn off the transistor Q2 to de-energize the X-ray contactor coil 103, thereby shutting off the X-ray system.

During the operation of the X-ray system, the peak-topeak ripple remaining after the filtering action of the high voltage capacitor 58 is effectively subtracted by the pulsating opposing voltage provided by the regulator tube 125. The regulator tube 125 is controlled to provide this pulsating subtracting kilovoltage by the AC feedback circuit of the capacitor 129 and the conductor 131 which connects the grid 128 of the impedance tube 125 to the kilovoltage supply conductor 52.

Any unselected kilovoltage change across the X-ray tube 43 as sensed by the kv. stabilizer 145 connected across the kv. selector resistor 139 initiates the kv. stabilizer to drive the control grid 128 of the regulator tube 125 to adjust the kilovoltage across the regulator tube 125 to maintain the supplied kilovoltage at the selected kv. setting. If the unselected change in the supplied kilovoltage is approximately 2.5 kv. or less, it is Within the adjustment range of the regulator tube 125 and the regulator tube 125 itself compensates for the unselected kilovoltage change. If the unselected kilovoltage change is more than approximately 2.5 kv. and the regulator tube 125 is driven beyond its 2.57.5 kv. adjustment range, then the kv. adjustment control circuit 150 is activated to cause the drive motor 38 to adjust the position of the variable voltage tap 35 until the kilovoltage on the supply conductor 52 is within the adjustment range of the regulator tube 125 and the kilovoltage across the regulator tube 125 approaches its 5 kv. nominal voltage.

The regulator tube acts as a Vernier and itself further compensates for the unselected change until the kilovoltage on the supply conductor 52 is that selected by the kilovoltage selector 139. i

If the kilovoltage selector 139 is positioned to a new selected kilovoltage setting within the 2.5-7.5 kv. adjustrnent range of the regulator tube, then the regulator tube 125 as driven by the stabilizer circuit changes the subtracting voltage to provide the selected kilovoltage setting. If the selected kilovoltage change is beyond the 2.5-7.5 adjustment of the regulator tube 125, then the adjustable voltage provider drive system 38 is actue ated by the kv. adjustment control circuit to reposition the movable tap 35 to bring the kilovoltage to near the selected kilovoltage and within the adjustment range provided by the regulator tube 125. The regulator tube 125 itself makes any further incremental adjustment necessary to increase or decrease the kilovoltage supplied to the X-ray tube to the selected value.

Selected changes in X-ray tube current are provided by the ma. selector 196. Unselected changes in the X-ray tube current are corrected by the ma. stabilizer 200. When an unselected X-ray tube current change occurs, the sensed voltage across the ma. selector resistor 196, Which is applied to the base of the transistor Q27, differs from the fixed reference voltage at the base of the transistor Q26 in the ma. stabilizer 200. This difference is amplified and applied to the base of the transistor Q28 to drive the variable impedance circuit 215 and effectively vary the impedance reflected to the secondary of the transformer 204. The change in reflected impedance in the transformer 204 changes the current in the filament supply conductor 79, and, hence, the filament current of the X-ray tube 43 so that the X-ray tube circuit current is automatically adjusted to the current level selected by the ma. selector resistor 196.

A change in the current in the X-ray tube circuit will also cause a change in the supply kilovoltage. This is because of a number of factors including the fact that the transformer 42 does have some DC resistance, that there is a change in the inductance values because there is always some imperfect coupling in the high voltage transformer 42, and there are series resistances in the circuit. Changes in X-ray tube voltage because of current changes are adjusted by the regulator tube 125 in the same manner as for other voltage changes. Thus, the present system compensates for changes in the supplied kilovoltage occurring because of load and line changes.

If during operation of the X-ray system, the kilovoltage supplied to the X-ray tube 43 exceeds certain limits, or the milliamperage in the X-ray tube circuit exceeds certain limits, then the kilovolt safety circuit 118 or the milliamperage safety circuit 117 respectively are activated to send a negative trigger pulse to the base of the transistor Q3 in the X-ray contactor coil control circuit 108 to de-ener-gize the X-ray contactor coil 103 and shut down the X-ray system 20. If the voltage across the impedance tube 125 exceeds certain limits, then the impedance tube safety circuit 116 similarly sends a negative trigger pulse to the base of the transistor Q3 in the X-ray contactor coil control 108 to cause the control circuit 108 to de-energize the X-ray contactor coil 103.

Whenever the X-ray contactor coil control circuit 108 receives such a negative trigger pulse from either the off switch 110, the X-ray timer 111, or the safety circuits 116, 118, then a positive signal pulse is sent from the emitter of the transistor Q2 via the conductor 190 to the kv. adjustment control circuit 150 to energize the decreasing relay coil 98. The energized relay coil 98 initiates operation of the voltage provider drive motor 38 to move the voltage tap 35 to the zero voltage position so that no voltage is provided to the high voltage transformer when the X-ray tube system 20 is turned oif.

Double X-ray tube system 270 Referring to FIGURE 7, a double X-ray tube system 270 is provided. The double X-ray tube system 270 is the single X-ray tube system shown in FIGURE 1, with two X-ray tube energizing circuits and two voltage and current control systems 271, 272.

In the double X-ray tube system 270, the secondary terminals 51, 54 of the high voltage transformer 42 are connected to high voltage supply conductors 52, 52 through the rectifiers 53, 56 and current limiting resistors 74, 74. The secondary terminals 47, 48 are connected the capacitors 129, 129' to the grids 128, 128' of the regulator tubes 125, 125 and to the high voltage supply conductors 52, 52'. Each of the regulator tubes 125, 125' assures a substantially pure DC kilovoltage supplied to the X-ray tubes 43, 43 respectively.

In the control systems 271, 272, milliampere control is provided for each of the X-ray tubes 43, 43' by ma. selector resistors 196, 196 respectively. Each of the X- ray tube control systems 271, 272 also have individual ma. stabilizers 200, 200, and ma. safety circuits 117, 117'. The ma. stabilizers 200, 200 and the ma. safety circuits 117, 117" are identical and operate exactly as the ma. stabilizer 200 and the ma. safety circuit shown in FIGURE 5. The ma. stabilizers 200, 200 maintain a constant stable X-ray tube current in their respective tube circuits despite differences in the currents selected for the X-ray tubes. The kv. selector resistors 139, 139 are ganged together (as indicated by the broken line 273) so that selective positioning of one selector 139 also positions the other selector 139' to provide the same kilovoltages supplied across their respective X-ray tubes 43, 43. Because of individual current selection by selector resistors 196, 196' and group kilovoltage selection by the ganged selector resistors 139, 139 both X-ray tube circuits operate at the same kilovoltage level although they may be operating at different X-ray tube currents.

Both control systems 271, 272 also have individual kv. stabilizers 145, and kv. safety circuits 118, 118 which operate exactly as the kv. stabilizer 145 and the kv. safety circuit 118 shown in FIGURE 3. Preferably, the negative 9 volts fixed reference voltages in the kv. stabilizers 145, 145 for both control systems 271, 272 are connected together to assure that the same fixed reference voltage is present in both stabilizer circuits. Since both X-ray tubes operate at the same kilovoltage level, then only the one major kilovoltage control circuit is required and is connected via a conductor 151 to the plate of the regulator tube 125 to respond to changes in the kilovoltage level beyond the adjustment limits of the regulator tube 125 and actuate the adjustable voltage provider drive motor 38 to bring the kilovoltage level back within the adjustment limits of the regulator tubes 125, 125'.

In the second X-ray tube control system 272, a regulator tube voltage sensing circuit conductor 151 is connected from the plate of the regulator tube 125 to the emitter of a PNP transistor Q38 in the regulator tubes safety circuit 116. As shown in FIGURE 2, the collector of the transistor Q38 is connected to the base of a PNP transistor Q39. The base of transistor Q38 is grounded. The base of the transistor Q39 is connected to a negative 18 volt power supply through a resistor 275 via a conductor 276. The collector of the transistor Q39 is connected to a negative 30 volt power supply and its emitter is connected to a ground point 277 through a resistor 278. A variation in the voltage across the regulator tube 125' results in a corresponding change in the bias at the base of the transistor Q39. This change in the bias of the base of the transistor Q39 causes a corresponding change in the conductivity of its emitter-collector circuit to produce a corresponding change in the current in the resistor 278. The voltage across the resistor 278 is thus directly related to a change in voltage across the regulator tube 125. The change in voltage across the resistor 278 is used by a ma. correction circuit 217' to correct the current in the conductor 50' for the current withdrawn from the X-ray tube circuit by the regulator tube voltage sensing circuit conductor 151'.

Ma. correction circuits 217, 217 are provided for the X-ray tubes 43, 43' respectively, and each are identical to that shown in FIGURE 6. The conductor 220' which withdraws the current Ir from the correction circuit 217' and hence from the circuit for the X-ray tube 43 is connected to conductor 279 between the resistor 278 and the emitter of the transistor Q39 in the regulator tubes safety circuit 116. Since the voltage across the resistor 278 varies in proportion to the voltage variations across the regulator tube 125', then a current Ir is in the conductor 220 which is identical to the current 11' in the conductor 220. When the current Ir, is deducted from the fixed current If provided by the fixed current providing circuit 222, the resultant current equals exactly the current Withdrawn from the X-ray tube circuit by the conductor 151 plus the error current la.

The regulator tube safety circuit 116 further includes a transistor Q40 having its base connected through a resistor 280 to the emitter of the transistor Q39. When the voltage across the regulator tube 125' rises to above permissible limits, for example, 10 kilovolts, then the voltage at the emitter of the transistor Q39 is such as to trigger the NPN transistor Q40 to cause it to conduct and send a negative trigger pulse through the conductor 115 to the base of the transistor Q3. The transistor Q3 then conducts to flop the flip-flop contactor coil control circuit 108 back to its OFF condition where the transistor Q2 is conducting and the transistor Q1 is cut off to de-energize the coil 103 and shut down the X-ray tube system 270. The outputs of all of the safety circuits 117, 117, 118, 118', are connected by their respective conductors 245, 245, 23s, 236' via the conductor 238 to the conductor 115 and transmit a negative trigger pulse to the base of the transistor Q3 in the X-ray contactor coil control circuit to thereby turn off the X-ray system 270 Whenever an overcurrent or overvoltage results.

With the present two X-ray tube system, it is possible for each X-ray tube to be operated on half-wave rectified kilovoltage and still individually select the X-ray tube currents. In addition, each of the individual X-ray tube energizing circuits can be provided with the high voltage filtering capacitors 58, 58' to smooth out the pulsating half-wave rectified kilovoltages provided by the secondary windings 47, 40 and the rectifiers 5'3, 56. This is done without creating unstable and oscillatory conditions within the X-ray tube circuits themselves. In addition, not only is it possible to use the high voltage filtering capacitors 58, 58 in each of the X-ray tube circuits, but it is also possible, by means of the regulator tubes 125, 125' to substantially eliminate all ripple in the D.C. kilovoltage provided to the X-ray tubes 43, 43'.

Although the invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and the scope of the invention as hereinafter claimed.

What is claimed is:

1. An X-ray tube system comprising:

(a) an X-ray tube;

(b) a voltage source;

(c) circuit means connecting said X-ray tube to said voltage source for supplying an energizing voltage to said X-ray tube;

(d) a variable voltage means connected in circuit with the voltage source in opposing relation to said voltage so that said energizing voltage equals the voltage source less the opposing variable voltage;

(e) first control means connected to said circuit means and to said variable voltage means to vary the opposing voltage according to variations in the voltage source so that said energizing voltage is substantially ripple free;

(f) second control means connected to said circuit means and to said variable voltage means to adjust the level of the opposing voltage of the variable voltage means in opposition to unselected changes in the level of the energizing voltage; and,

(g) voltage adjustment control means connected to said circuit means and to said variable voltage means, said adjustment control means being responsive to changes in the level of the opposing voltage beyond a predetermined adjustment range and adjusting the level of the energizing voltage to bring the level of the necessary opposing voltage within said predeter- 22 mined adjustment range whereby said voltage adjustment control means adjusts for large unselected changes in the energizing voltage level and the second control means adjust for small unselected changes in energizing voltage level.

2. An X-ray system for supplying and maintaining a selected level of DC voltage to an X-ray tube from an AC source, said system comprising:

(a) a variable voltage provider having a voltage input for connection to the AC source, a voltage output and a voltage control for adjusting the voltage level at the voltage output;

(b) high voltage supply conductor means for supplying an energizing voltage to the X-ray tube;

(c) circuit means connecting the supply conductor means to the voltage output of said variable voltage provider, said circuit means including rectifier means for providing a rectified voltage to be supplied to the X-ray tube by the supply conductor means, and capacitive means for filtering the rectified voltage;

(d) a voltage subtracting device connected in circuit with the circuit means and the X-ray tube so that a voltage appearing across the voltage subtracting device is subtracted from the rectified voltage provided by said circuit means;

(c) said voltage subtracting device including control means for varying between fixed limits the voltage appearing across the subtracting device;

(f) an AC feedback means connected to said supply conductor and to said control means for varying the subtracting voltage according to ripple variations in the rectified and filtered voltage so as to eliminate such ripple variations in the energizing voltage supplied to the X-ray tube;

(g) an energizing voltage sensing means having an input connected across the X-ray tube and an output connected to the subtracting device control means, said voltage sensing means sensing the energizing voltage supplied to the X-ray tube and supplying a control signal to the voltage subtracting device control means to change the level of the subtracting voltage in response to unselected changes in the level of the energizing voltage within a predetermined adjustment range from a predetermined level of energizing voltage supplied to the X-ray tube; and,

(h) voltage adjustment con-trol means having an input connected across said voltage subtracting device and an output connected to the voltage control of the variable voltage provider, said voltage adjustment control means being responsive to the voltage across said voltage subtracting device and actuating the voltage control of the voltage provider When the opposing voltage level changes beyond said predetermined adjustment range to adjust the voltage at the voltage output until the energizing voltage is within said predetermined adjustment range.

3. The system of claim 2 including:

(i) X-ray tube current sensing means connected in circuit with the circuit means and the voltage subtracting device; and,

(j) a current correction circuit means connected to the X-ray tube current sensing means, said current correction circuit means correcting the current in the current sensing means for the current in the energizing voltage sensing means and the current in the input of the voltage adjustment control means.

4. An X-ray system for supplying a DC voltage to an X-ray tube from an AC voltage source, said system comprising:

(a) a step-up high voltage transformer having a primary winding for connection to the voltage source, a high voltage secondary winding having first and second output terminals and a center tap terminal located intermediate between the first and second winding terminals;

(b) a grounded conductor connected at one end to said intermediate voltage tap terminal and grounded at its other end;

(c) a high voltage supply conductor connected to the first and second winding terminals for Supplying a high voltage to the X-ray tube;

(d) first and second rectifiers connected to said first and sec-nd terminals respectively and to the high voltage supply conductor for supplying a full wave rectified voltage to the X-ray tube;

(e) a capacitive filter circuit connected to the high voltage supply conductor and to the grounded conductor adjacent said intermediate terminal;

(f) a voltage subtracting device interposed in said grounded conductor with its anode and cathode elements connected in series with the X-ray tube so that the voltage across said voltage subtracting device is subtracted from the voltage across the secondary of the high voltage transformer, said voltage subtracting device having a control element for controlling the magnitude of subtracted voltage;

(g) an AC feedback circuit including a capacitor connected to said high voltage supply conductor and to the control element to cause the voltage across the subtracting device to vary according to ripple variations in the negative high voltage potential supplied to the high voltage supply conductor;

(h) a variable impedance current selector interposed in the grounding conductor between the voltage subtracting device and a ground potential point, the impedance of the current selector being variable to selectively adjust the current in the X-ray tube circuit;

(i) a filament voltage supply circuit connected to a filament of the X-ray tube for supplying energizing current thereto;

(j) an X-ray tube current stabilizer circuit connected to the filament supply circuit and across the current selector, said X-ray tube stabilizer circuit being responsive to the voltage across the current selector and controlling the filament voltage supply circuit to increase or decrease the current supplied to the filament in response to decreases or increases respectively of the current in the X-ray tube circuit to maintain a preselected level of current in the X-ray tube circuit;

(k) an X-ray tube voltage sensing circuit connected across the X-ray tube, said voltage sensing circuit including a variable impedance high voltage selector;

(1) a high voltage stabilizer control circuit having an input connected across the high voltage selector and an output connected to the control element of the voltage subtracting device, said high voltage stabilizer control circuit being responsive to an increase or decrease from a predetermined level of the high voltage supplied to the X-ray tube to adjust the bias on the control element of the voltage subtracting device to increase or decrease the voltage across the voltage subtracting device in response to increases or decreases respectively in the energizing voltage supplied to the X-ray tube at said preselected voltage level; and,

(In) a current correction circuit means connected to the grounding conductor between the current selector and the secondary winding, said current correction circuit means correcting the current in the grounding conductor for the current in the X-ray tube voltage sensing circuit so that actual tube current is present in the current selector.

5. The device of claim 4 including:

(n) a current measuring and indicating device interposed in the grounding conductor between the current correction circuit means and the grounded end of the conductor.

24 6. An X-ray system for supplying a DC voltage to an X-ray tube from an AC voltage source, said system comprising:

(a) a variable voltage provider including a selector coil, input terminals for connecting the coil across the AC voltage source, a first output terminal connected to one end of the coil and a movable second output terminal positionable along the coil between its ends;

(b) a high voltage step-up transformer having a primary winding and a high voltage secondary winding, said primary winding being connected to the output terminals of the variable voltage provider, and said secondary winding having first and second output terminals and an intermediate voltage tap located intermediate between the secondary winding terminals;

(c) a grounding conductor having one end connected to said intermediate voltage tap terminal and its other end grounded;

(d) a high voltage supply conductor connected to said first and second winding terminals for supplying a high voltage to the X-ray tube;

(e) first and second rectifiers connected to said first and second terminals respectively and to the high voltage supply conductor for supplying a full Wave rectified voltage to the X-ray tube;

(f) a capacitive filter circuit connected to the high voltage supply conductor and to the grounding conductor adjacent said intermediate tap;

(g) a voltage subtracting device interposed in said grounding conductor and in series with the X-ray tube so that the voltage across said voltage subtracting device is subtracted from the voltage across the secondary of the high voltage transformer, said voltage subtracting device having a control element for controlling the magnitude of subtracted voltage;

(h) an AC feedback circuit including a capacitor connected to said high voltage supply conductor and to the control element to cause the voltage across the voltage subtracting device to vary according to ripple variations in the high voltage potential supplied by the high voltage supply conductor;

(i) a variable impedance X-ray tube current selector interposed in the grounding conductor between the voltage subtracting device and a ground potential point, the impedance of the current selector being variable to selectively adjust the current in the X-ray tube circuit;

(j) a filament voltage supply circuit connected to a filament of the X-ray tube for supplying energizing current thereto;

(k) an X-ray tube current stabilizer circuit connected to the filament supply circuit and across the X-ray tube current selector, said X-ray tube current stabilizer circuit being responsive to the voltage across the X-ray tube current selector and controlling the filament voltage supply circuit to increase or decrease the current supplied to the filament in response to decreases or increases respectively of the current in the X-ray tube circuit to maintain a preselected level of the current in the X-ray tube circuit;

(1) an X-ray tube voltage sensing circuit connected across the X-ray tube, said voltage sensing circuit including a variable impedance high voltage selector;

(m) a high voltage stabilizer control circuit having an input connected across the high voltage selector and an output connected to the control element of the voltage substracting device, said high voltage stabilizer control circuit being responsive to an increase or decrease from a predetermined level of the high voltage supplied to the X-ray tube to adjust the 'bias on the control element of the voltage subtracting device to increase or decrease the subtracted voltage in response to increases or decreases respectively in the energizing voltage supplied to the X-ray tube at said preselected voltage level;

(n) a variable voltage selector drive motor including forward and reverse rotation windings and a rotor which is induced to rotate in a forward rotational direction when the forward winding is energized and is induced to rotate in a reverse rotational direction when the reverse winding is energized;

() means operatively connecting said rotor to the movable voltage terminal so that when the forward winding is energized the rotor moves the movable voltage terminal along the coil in a voltage increasing direction away from the first output voltage terminal and when the reverse winding is energized the rotor moves the movable voltage terminal in a voltage decreasing direction toward the first output terminal;

(p) a voltage adjustment control circuit having an input connected across the voltage subtracting device and an output connected to said forward and reverse windings, said voltage adjustment control circuit being responsive to the voltage across the voltage subtracting device and energizing said forward winding when the subtracted voltage increases beyond a first predetermined voltage level limit, and said voltage selector control circuit energizing said reverse Winding when the subtracted voltage decreases beyond a second predetermined voltage level limit, said first and second predetermined limits defining an adjustment range of the voltage subtracting device for maintaining the energizing voltage at said preselected voltage level, whereby said voltage subtracting device adjusts for changes in the level of the energizing voltage within said adjustment range and said voltage adjustment control circuit adjusts for changes in the level of. the energizing voltage beyond said adjustment range.

7. The system of claim 6 including;

(q) a current correction circuit connected to the grounding conductor between said current selector and the input of the voltage selector control circuit, said current correction circuit correcting the current in the current selector for the current in the X-ray tube voltage sensing circuit and for the current in the input of the voltage selector control circuit so that actual X-ray tube current appears in said current selector.

-8. The system of claim 7, including:

(r) a current sensing and indicating meter interposed in said grounding conductor between said current correction circuit and the grounded end of the grounding conductor.

9. The system of claim 6 including:

(q) switching means interposed between the input terminals of the variable voltage provider and the AC voltage source;

(r) switching means control connected to the switching means for selectively operating said switching means between ON and OFF positions connecting and disconnecting the selector coil across the AC voltage source in response to ON and OFF signals respectively introduced to an input of the switching means control;

(s) circuit means connecting an output of the switching means control to a second input of the voltage adjustment control circuit for supplying a control signal to the voltage adjustment control circuit each time an OFF signal is introduced to the input of the switching means control, said voltage adjustment control circuit energizing said reverse winding in response to said control signal to substantially reduce the energizing voltage supplied to the X-ray tube.

10. An X-ray system for supplying and maintaining a selective level of energizing voltage to an X-ray tube from a voltage source, said system comprising:

(a) an X-ray tube;

(b) high voltage supply conductor means connected to the X-ray tube for supplying an energizing voltage to the Xaray tube;

(c) circuit means connecting the supply conductor mean-s to a voltage source;

(d) a voltage regulating device connected in circuit with the circuit means and the X-ray tube so that a voltage appearing across the voltage regulating device is in opposing relation to the source voltage;

(c) said voltage regulator device including control means for varying between fixed limits the opposing voltage appearing across the regulator device;

(f) a voltage selection and sensing circuit means connected across the X-ray tube, said voltage selection and sensing circuit means including a variable impedance device;

(g) a voltage stabilizer having an input connected across the variable impedance device and an output connected to the control means of the voltage regulator device, said voltage stabilizer including:

(i) a difference amplifier connected to the input of the voltage stabilizer and to a predetermined reference voltage, said difference amplifier providing a ditference voltage signal in response to a difference between the predetermined reference voltage and the voltage appearing across the variable impedance device, said difference amplifier supplying this diiference voltage signal through the output of the voltage stabilizer to the control means of the voltage regulating device so as to adjust the level of the opposing voltage in relation to the source voltage to ad just the level of the energizing voltage until the voltage across the variable impedance device is equal to the fixed predetermined reference voltage.

(h) said circuit means including a variable voltage provider having a voltage input which is connected to the voltage source, a voltage output which is con nected to the supply conductor means, and a voltage control for adjusting the voltage level at the voltage output; and,

(i) voltage adjustment control means having an input connected across said voltage regulating device and an output connected to the voltage con-trol of the variable voltage provider, said voltage adjustment control means being responsive to the voltage across said regulator device and actuating the voltage control of the voltage provider when the opposing voltage level changes beyoud a predetermined adjustment range to adjust the voltage level at the voltage output until the energizing voltage is within said predetermined adjustment range.

11. An X-ray system for supply and maintaining a selected level of energizing voltage to an X-ray tube from a voltage source, said system comprising:

(a) an X-ray tube;

(b) high voltage supply conductor means connected to the X-ray tube for supplying an energizing voltage to the X-ray tube;

(c) circuit means for connecting the supply conductor means to a voltage source, said circuit means including:

(i) a variable voltage providing means having a voltage input which is connected to the voltage source;

(ii) a voltage output which is connected to the supply conductor means; and,

(iii) a voltage control for adjusting the voltage level at the voltage output;

((1) a voltage regulating device connected in circuit with the circuit means and the X-ray tube so that a voltage appearing across the voltage regulating device is in opposing relation to the output voltage; (c) said voltage regulator device including control means for varying the opposing voltage appearing means to a voltage source, said circuit means including:

(i) a variable voltage providing means having a across the regulator device between fixed limits of a predetermined voltage range;

(f) a voltage selecting and sensing circuit means connected across the X-ray tube, said voltage selecting across the regulator device between fixed limits of and sensing circuit means including a variable ima predetermined voltage range; pedance device;

(f) an energizing voltage selecting and sensing circuit (g) a voltage stabilizer having an input connected means connected across the X-ray tube for preselectacross the variable impedance device and an output ing the energizing voltage supplied to the X-ray tube; connected to the control means of the voltage regu- (g) a voltage control circuit connected to said voltage lator device, said voltage stabilizer including:

selecting and sensing circuit and to the control means (i) a difference amplifier connected to the input of the voltage regulator device to adjust the level of of the voltage stabilizer and to a predetermined the opposing voltage over a predetermined voltage reference voltage, said difference amplifier prorange to maintain a preselected energizing voltage viding a difference voltage signal in response to supplied to the X-ray tube; a difference between the predetermined refer- (h) voltage adjustment control means including: ence voltage and the voltage appearing across (i) a differential circuit means having a voltage the variable impedance device, said difference sensing input connected across said voltage reguamplifier supplying this difference voltage signal lator device for determining the direction of any through the output of the voltage stabilizer to change of the opposing voltage level, a control the control means of the voltage regulating deinput, and an output connected to the voltage vice so as to adjust the level of the opposing control of the variable voltage provider for advoltage in relation to the output voltage to ad justing the level of the output voltage in a direcjust the level of the energizing voltage until the tion as indicated by said voltage sensing input voltage across the variable impedance device is for the duration of a control signal introduced effectively equal to the fixed predetermined refat said control input; erence voltage;

(ii) a switching circuit having an input for re- (h) voltage adjustment control means including:

ceiving ON and OFF trigger signals and an out- (i) a differential circuit means having a voltage put connected to the control input to introduce sensing input connected across said voltage rega control signal to said control input when an ulator device for determining the direction of ON signal is introduced at the input of the any change of the opposing voltage level, a conswitching circuit and to discontinue the control tral input, and an output connected to the voltsignal when an OFF signal is introduced to the age control of the variable voltage provider for input of the switching circuit; adjusing the level of the output voltage in the (iii) a first voltage range determining circuit havdirection indicated by said voltage sensing input ing an input connected across said voltage regufor the duration of a control signal introduced lating device and an output connected to the at said control input; input of the switching circuit, said first voltage (ii) a switching circuit having an input for rerange determining circuit sending an ON trigger ceiving ON and OFF trigger signals and an signal to the input of the switching circuit whenoutput connected to the control input to introever the voltage across the voltage regulator dedllee 1 Control Signal to Said C ntr l input Wh n device goes in either direction beyond said prean ON signal is introduced at the input of the determined voltage range; and, switching circuit and to discontinue the control (iv) a second voltage range determining circuit Signal When an OFF Signal is ced at the having an input connected across said voltage input of the Switching circuit; regulating device and an output connected to (iii) a first voltage range determining circuit having the input of the switching circuit, said second an input connected across said voltage regulating voltage range determining circuit sending an deviee and an Output Connected to the input of OFF trigger signal to the input of the switchthe Switching circuit, Said first Voltage range ing circuit whenever the level of the opposing 5O determining Circuit Sending an ON trigger Signal voltage returns to within said predetermined t0 the i put Of the switching circuit whenever 1t range the voltage across the voltage regulator device 12. An X-ray system for supplying and maintaining 21 goes in either direction beyond Said predeterselected level of energizing voltage to an X-ray tube from mined Voltage range;

a voltage source, said system comprising: a seeend Voltage range determining Circuit (a) an X- ay b mg an input connected across said voltage regu- (b) high voltage supply conductor means connected lating device and an Output Connected t the to the X-ray tube for upplying an energizing voltage input of the switching circuit, Said second voltage to h X- b range determining circuit sending an OFF trig- (c) circuit means for connecting the supply conductor g Signal to the input of the Switching Circuit whenever the level of the opposing voltage returns to within said predetermined voltage range. 13. The system of claim 12 wherein said second voltage range determining circuit sends the OFF signal to the input of the switching circuit when the opposing voltage level is substantially at the middle of said predetermined voltage range.

14. The system of claim 13 wherein the switching circuit is a flip-flop circuit.

15. An X-ray system for supplying a DC voltage to an X-ray tube from an AC voltage source, said system comprlsmg:

(a) a variable voltage provider including a selector coil,

input terminals for connecting the coil across the AC voltage source, a first output terminal connected to voltage input which is connected to the voltage source; 6 (ii) a voltage output which is connected to the supply conductor means; and, (iii) a voltage control for adjusting the voltage level at the voltage output;

(d) a voltage regulating device connected in circuit with the circuit means and the X-ray tube so that a voltage appearing across the voltage regulating device is in opposing relation to the voltage output;

(e) said voltage regulator device including control means for varying the opposing voltage appearing 29 one end of the coil and a movable second output terminal positionable along the coil between its ends;

(b) a high voltage step-up transformer having a primary winding and a high voltage secondary winding, said primary winding being connected to the output terminals of the variable voltage provider, and said secondary Winding having first and second output terminals and a voltage tap located intermediate between the secondary winding terminals;

(c) a grounding conductor having one end connected to the intermediate voltage tap terminal and its other end grounded;

(d) a high voltage supply conductor connected to said first and second winding terminals for supplying a high voltage to the X-ray tube;

(e) first and second rectifiers connected to said first and second terminals respectively and to the high voltage supply conductor for supplying a full wave rectified voltage to the X-ray tube;

(f) a capacitive filler circuit connected to the high voltage supply conductor and to the grounding conductor adjacent said intermediate tap;

(g) a voltage subtracting device interposed in said grounding conductor and connected in series with the X-ray tube so that the voltage across said voltage subtracting device is subtracted from the voltage across the secondary of the high voltage transformer, said voltage subtracting device having a control element for controlling the magnitude of subtarcted voltage; a

(h) an AC feedback circuit including a capacitor connected to said high voltage supply conductor and to the control element to cause the voltage across the voltage subtracting device to vary according to ripple variations in the high voltage potential supplied by the high voltage supply conductor;

(i) a variable impedance X-ray tube current selector interposed in the grounding conductor between the voltage subtracting device and a ground potential point, the impedance of the current selector being variable to selectively adjust the current in the X-ray tube circuit; Y

(j) a filament voltage supply circuit connected to a filament of the X-ray tube for supplying energizing current thereto;

(k) an X-ray tube current stabilizer circuit connected to the filament supply circuit and across the X-ray tube current selector, said X-ray tube current stabilizer circuit being responsive to the voltage across the X-ray tube current selector and controlling the filament voltage supply circuit to increase or decrease the current supplied to the filament in response to decreases or increases respectively of the current in the X-ray tube circuit to maintain a preselected level of the current in the X-ray tube circuit;

(1) an X-ray tube voltage sensing circuit connected across the X-rayntube, said voltage sensing circuit including a variable impedance high voltage selector;

(m) a high voltage stabilizer control circuit having an input connected across the high voltage selector and an out-put connected to the control element of the voltage subtracting device, said high voltage stabilizer control circuit being responsive to an increase or decrease from a predetermined level of the highvoltage supplied to the X-ray, tube to adjust the bias on the control element of the voltage, subtracting device to increase or decrease the subtracted voltage in response to increases or decreases respectively in the energizing voltage supplied to the X-ray tube at said preselected voltage level;

(n) a variable voltage selector drive motor including forward and reverse rotation windings and a rotor I which is induced to rotate in a forward rotational 7 direction when the forward winding is energized and is induced to rotate in a reverse rotational direction when the reverse Winding is energized;

(0) means operatively connecting said rotor to the movable voltage terminal so that when the forward winding is energized the rotor moves the movable voltage terminal along the coil in a voltage increas: ing direction away from the first output voltage terminal and when the reversed winding is energized the rotor moves the movable voltage terminal in a voltage decreasing direction toward the first output terminal;

(p) a voltage adjustment control circuit comprising:

(i) first and second relays controlling energization of said forward and reverse windings respective ly, said relays including coils each having one end connected to an energy source and causing energization of its respective winding when its respective coil is connected across said energy source;

(ii) .a differential amplifier having first and second transistors connect-ed in a differential amplifier arrangement, the emitters of the transistors being connected to a switching conductor, the collectors of the first and second transistor being connected to the first and second relay coils respectively, the base of one transistor receiving a fixed reference signal, the base of the other transistor being connected across the voltage subtracting device and receiving a sensed voltage signal from the voltage subtracting device, the transistors being arranged such that when the sensed voltage signal exceeds the fixed reference signal the second transistor is more conductive than the first transistor and when the sensed voltage signal is less than the fixed reference signal the first transistor is more conductive than the second transistor, neither of said tran sistors being sufficiently conductive to energize their respective relay coils until said switching conductor is connected to the energy source to place the coil across the energy source;

(iii) a switching circuit having an input for receiving ON and OFF trigger signals and an output connected to said switching conductor to connect it to the energy source when an ON signal is introduced to the input of the switching circuit and to disconnect the switching conductor from the energy source when an OFF signal is introduced to the input;

(iv) a first voltage sensing circuit having an input connected across the voltage subtracting device for receiving the sensed voltage signal and an output connected to the input of the switching circuit, said first voltage sensing circuit sending an ON trigger signal to the input of the switching circuit whenever the sensed voltage goes beyond the limits of a predetermined voltage range; and,

(v) a second voltage sensing circuit having an input for receiving the sensed voltage signal and an output connected to the input of the switching circuit, said second voltage sensing circuit sending an OFF trigger signal to the input of the switching circuit whenever the sensed voltage returns to within said predetermined voltage range whereby the reverse winding is energized when the sensed voltage increases beyond one limit of the predetermined voltage range and the forward winding is energized when the sensed voltage decreases beyond the other limit of the predetermined voltage range, the limits of the sensed predetermined voltage range corresponding to an adjustment range of thevoltage subtracting device for maintaining the energizing voltage at said preselected voltage level, and said voltage subtracting device adjusts for changes in the level of the energizing voltage within said adjustment range and said voltage adjustment control circuit adjusts for changes in the level of the energizing voltage beyond said adjustment range.

16. A system comprising:

(a) first and second relay coils each connected to one side of an energy source;

(b) a differential amplifier having first and second transistors connected in a differential amplifier arrangement, the emitters of the transistors being connected to a conductor, the collectors of the first and second transistors being connected to said first and second relay coils respectively, the base of one transistor receiving a fixed reference signal, the base of the other transistor receiving a sensed voltage signal, the transistors being arranged such that when the sensed voltage signal exceeds the fixed reference signal, the first transistor is more conductive than the second transistor and when the sensed voltage signal is less than the fixed reference signal, the second transistor is more conductive than the first transistor, but neither of said transistors being sufficiently conductive to energize their respective relay coils until said conductor is connected to the other side of the energy source;

(c) a switching circuit having an input for receiving ON and OFF trigger signals and an output connected to said conductor to connect it to the energy source when an ON signal is introduced at the input of the switching circuit and to disconnect the conductor from the energy source when an OFF signal is introduced at its input;

(d) a first voltage sensing circuit having an input for receiving the sensed voltage signal and an output connected to the input of the switching circuit, said first voltage sensing circuit sending an ON trigger signal to the input of the switching circuit whenever the sensed voltage goes above or below a predetermined voltage range; and,

(e) a second voltage sensing circuit having an input for receiving the sensed voltage signal and an output connected to the input of the switching circuit, said second voltage sensing circuit sending an OFF trigger signal to the input of the switching circuit whenever the sensed voltage returns to within said predetermined voltage range.

17. A system comprising:

(a) first and second relay means each connected to one side of an energy source;

(b) a differential amplifier having first and second transistors connected in a differential amplifier arrangement, the emitters of the transistors being connected to a conductor, the collectors of the first and second transistors being connected to said first and second relay means respectively, the base of One transistor receiving a fixed reference signal, the base of the other transistor receiving the sensed voltage signal, the transistors being arranged such that when the sensed voltage signal exceeds the fixed reference signal, the first transistor is more conductive than the second transistor and when the sensed voltage signal is less than the fixed reference signal the second transistor is more conductive than the first transistor, but neither of said transistors being sufficiently conductive to energize their respective relay means until said conductor is connected to the other side of the energy source;

() a switching circuit having an input for receiving ON and OFF trigger signals and an output connected to said conductor to connect it to the energy source when an ON signal is introduced at the input of the switching circuit and to disconnect the conductor 75 32; from the energy source when an OFF signal is introduced at its input;

(d) second and third differential amplifiers each having an output connected to the input of the switching circuit, a first input for receiving said sensed voltage signal and a second input for connection to a fixed reference signal;

(e) said second and third differential amplifiers providing ON trigger pulses at their outputs when the sensed voltage signal goes in either direction beyond a predetermined voltage range and OFF trigger pulses when the sensed voltage signal goes from beyond said predetermined voltage range to within the voltage range;

(f) first and second diodes interposed between the input of the switching circuit and the outputs of the second and third differential amplifiers, said first diodes being poled oppositely of said' first diode so that the ON trigger pulses produced only by said second differential amplifier reach the input of the switching circuit and so that the OFF trigger pulses produced only by said third differential amplifier reach the input of the switching circuit.

18. The system of claim 17 wherein said third differential amplifier produces an OFF trigger pulse when the sensed voltage is substantially at the mid-point of the voltage range when returning from beyond the voltage range.

19. The, system of claim 17 wherein said switching circuit is a flip-flop circuit.

20. An X-ray system comprising:

(a) a high voltage transformer having a primary winding for connection to a voltage source, a high voltage secondary winding having first and second output terminals and a center tap terminal located interlmediate between the first and second output termina s;

(b) an X-ray tube;

(c) a high voltage supply conductor connected to the first and second output terminals and to the X-ray tube for supplying high voltage to the X-ray tube;

(d) first and second rectifiers connected to the first and second output terminals respectively and to the high voltage supply conductor for supplying a full wave rectified voltage to the X-ray tube;

(e) a variable impedance current selector;

(f) conductor means connecting said curent selector to the center tap terminal and to ground potential;

(g) a current adjustment means connected to the X-ray tube for adjusting the passage of current therethrough;

(h) a current stabilizer means connected across the current selector and to the current adjustment means, said current stabilizer maintaining a predetermined current in the X-ray tube as selected by the current selector; and v (i) a current correction circuit means connected to the conductor means between the transformer center tap terminal and the current selector, said current correction circuit means correcting the current in the conductor means so that the current appearing in the current selector is the actual current drawn by the X-ray tube.

21. The combination of claim 20' wherein the current stabilizer includes a differential amplifier having first and second inputs and an output, one of said inputs being connected across said current selector, the other of said inputs being connected across a predetermined reference voltage, and the output of the different amplifier being connected to the current adjustment means of the X-ray tube whereby when the voltage appearing across the current selector is introduced to said first input and compared against said reference voltage, the differential amplifier provides a difference signal to the current adjustment means to adjust the X-ray tube current if the voltage 

1. AN X-RAY TUBE SYSTEM COMPRISING: (A) AN X-RAY TUBE; (B) A VOLTAGE SOURCE; (C) CIRCUIT MEANS CONNECTING SAID X-RAY TUBE TO SAID VOLTAGE SOURCE FOR SUPPLYING AN ENERGIZING VOLTAGE TO SAID X-RAY TUBE; (D) A VARIABLE VOLTAGE MEANS CONNECTED IN CIRCUIT WITH THE VOLTAGE SOURCE IN OPPOSING RELATION TO SAID VOLTAGE SO THAT SAID ENERGIZING VOLTAGE EQUALS THE VOLTAGE SOURCE LESS THE OPPOSING VARIABLE VOLTAGE; (E) FIRST CONTROL MEANS CONNECTED TO SAID CIRCUIT MEANS AND TO SAID VARIABLE VOLTAGE MEANS TO VARY THE OPPOSING VOLTAGE ACCORDING TO VARIATIONS IN THE VOLTAGE SOURCE SO THAT SAID ENERGIZING VOLTAGE IS SUBSTANTIALLY RIPPLE FREE; (F) SECOND CONTROL MEANS CONNECTED TO SAID CIRCUIT MEANS AND TO SAID VARIABLE VOLTAGE MEANS TO ADJUST THE LEVEL OF THE OPPOSING VOLTAGE OF THE VARIABLE VOLTAGE MEANS IN OPPOSITION TO UNSELECTED CHANGES IN THE LEVEL OF THE ENERGIZING VOLTAGE; AND, (G) VOLTAGE ADJUSTMENT CONTROL MEANS CONNECTED TO SAID CIRCUIT MEANS AND TO SAID VARIABLE VOLTAGE MEANS, SAID ADJUSTMENT CONTROL MEANS BEING RESPONSIVE TO CHANGES IN THE LEVEL OF THE OPPOSING VOLTAGE BEYOND A PREDETERMINED ADJUSTMENT RANGE AND ADJUSTING THE LEVEL OF THE ENERGIZING VOLTAGE TO BRING THE LEVEL OF THE NECESSARY OPPOSING VOLTAGE WITHIN SAID PREDETERMINED ADJUSTMENT RANGE WHEREBY SAID VOLTAGE ADJUSTMENT CONTROL MEANS ADJUSTS FOR LARGE UNSELECTED CHANGES IN THE ENERGIZING VOLTAGE LEVEL AND THE SECOND CONTROL MEANS ADJUST FOR SMALL UNSELECTED CHANGES IN ENERGIZING VOLTAGE LEVEL. 